1. Field:
The invention is in the field of apparatus used by scuba divers, and particularly relates to apparatus used to indicate the extent of gas accumulation in the body of a scuba diver.
2. State of the Art:
A column of fresh water 33 feet high or sea water 32 feet high exerts one atmosphere of pressure. Since the water surface is already at one atmosphere pressure, the pressure at a depth of 33 feet in fresh water is two atmospheres, and the pressure increases by one atmosphere for every 33 feet further that one descends. Thus, at a depth of 100 feet, the pressure exerted on a diver will be about four atmospheres.
As a diver descends and the pressure on his body increases, any gases in air chambers, such as his lungs, are compressed. At a depth of 33 feet, where the pressure is two atmospheres, the volume of air in his lungs is compressed to one-half the volume it will fill at sea level. Similarly, at 100 feet, where the pressure is four atmospheres, the air will be compressed to one-fourth of its volume at sea level.
In order to counteract the effects of pressure on a diver as he descends, and allow him to breathe properly, he is supplied with required quantities of air from tanks of compressed air. However, even though this permits the diver's lungs to maintain their normal volume, and the diver suffers no ill effects, the pressure of the air in his lungs is increased in proportion to the depth at which he is diving.
Air is made up primarily of oxygen (about 20%) and nitrogen (about 80%). At sea level, approximately one liter of nitrogen is dissolved in an average person's body. Nitrogen is about five times more soluble in fat as in water so that more than half of the nitrogen is dissolved in body fats, even though fats only make up about 15% of the body.
Due to the increased pressure of air in his lungs, the amount of oxygen and nitrogen which dissolves in a diver's body also increases. Nitrogen is not metabolized by the body, so it remains dissolved in the body to an extent dependent on the external pressure. Oxygen, on the other hand, is metabolized and thus is not generally a problem when a diver breathes compressed air. For each increase in pressure of one atmosphere, an additional liter of nitrogen will dissolve in his body. Thus, at 33 feet a diver will have two liters of nitrogen dissolved in his body; at 100 feet his body will contain four liters. However, the increased nitrogen does not dissolve in a diver's body instantly. Furthermore, the nitrogen dissolves at different rates in different parts of the diver's body. Water in the diver's body becomes saturated with nitrogen in about one hour, whereas fat, which requires much more nitrogen before it is saturated, and also has a poor blood supply to carry the nitrogen, reaches saturation only after several hours. Thus, several hours are required before the body becomes saturated with nitrogen as all of the tissues in the body come into equilibrium with the gas pressure in the diver's lungs.
Unless the diver is at a depth of about 130 feet or more, such that he may begin to develop nitrogen narcossis, he will generally suffer no ill effects from the increased nitrogen dissolved in his body, as long as he remains submerged. However, as he ascends, the pressure on his body decreases and excess nitrogen is liberated from his body fluids and tissues. If the ascent is too rapid, actual bubbles of nitrogen will form. Bubbles forming in the brain, spinal cord, or peripheral nerves can cause paralysis or convulsions, or other effects. Bubbles in the joints or muscles can cause severe pain. Nitrogen bubbles in the respiratory system can cause difficulty in breathing and heavy coughing. In any event, the experience is unpleasant and could result in permanent injury. To avoid these effects a diver must either ascend slowly enough to allow the excess nitrogen to be expelled slowly from his body without bubble formation, or he must ascend before too much extra nitrogen has dissolved in his body.
The amount of nitrogen which actually dissolves in a diver's body is a function of the depth to which he descends, and the length of time he remains there.
The U.S. Navy has published numerous tables indicating safe procedures to be used to avoid formation of nitrogen bubbles in a diver's body. It has been discovered that the mere fact that excess nitrogen is present in a diver's body does not mean that nitrogen bubbles will necessarily form. Nitrogen can be "supersaturated" in a diver's body so that only an insignificant quantity of nitrogen bubbles will form even where the quantity of dissolved nitrogen is greater than the amount the body would normally hold at a given pressure. The U.S. Navy tables allow some excess nitrogen to remain in a diver's body during his ascent. However, it should be noted that the various tables are based on an "average" diver. A particular diver may be more or less susceptible to formation of nitrogen bubbles in his body.
A diver using the Navy Tables must take into account the fact that when he surfaces, his body will be supersaturated with nitrogen to some extent. Thus, a diver making a second dive shortly after completing the first must adjust for the excess nitrogen in his body. The Navy has published a diving manual containing a set of tables for use where a diver makes repetitive dives. One of these tables sets forth various times at which a diver can remain at specified depths without requiring decompression. When a diver intends to make a second or subsequent dive, a second table is used to account for the amount of time a diver spends at the surface between dives. The second table leads into a third table which tells a diver his "residual nitrogen times" at various depths. These residual times are the times a diver must assume he already has spent at a given depth when he starts a repetitive dive. The U.S. Navy has also published various tables to be used where decompression is necessary. Depending upon the circumstances, these tables set forth periods of time which a diver must spend decompressing at various depths as he ascends from a dive made at a deeper depth.
A diver needs to have access to the information contained in some of these tables during each dive he makes. The most common method of carrying this information is on a plastic card having the tables printed thereon. Typically, a plastic card used for this purpose measures between four to six inches wide and between eight to ten inches long, and is provided with a hole through which a chain or cord passes for use in securing the card to the diver. The problem with use of a card is that it is somewhat awkward to carry since it tends to flutter as a diver swims. It is also somewhat awkward to use because it is difficult to move from one table to another without losing one's place, thus making it likely that one will make a mistake and perhaps stay down too long. It is particularly difficult for a diver to use when he realizes he has been down too long and begins to panic.
One device that has partially solved these problems is disclosed in U.S. Pat. No. 3,058,653. This device is a circular slide-rule type "computer" which shows much of the information in windows, excluding much of the unwanted information from view. However, the device is designed to be carried similarly to the cards, and its size and shape makes it no more convenient to carry than a card. Also, the device still uses tables to convey some required information, with the attendant risk of slipping a line as one reads across, thus reading the wrong information.